Classes of Steel

Steels vary greatly but the major classes are carbon steels, low-alloy steels (up to 8% alloying elements, i.e. tool steels), and high-alloy steels (more than 8% alloying elements, i.e. stainless steels).

In carbon steels, the carbon content may range from 0.015% to 2%.

The steel that was used for the Golden Gate Bridge, for instance, is carbon steel with the following average chemical composition: C: 0.81% (0.85), Mn: 0.66%, P: 0.026% (0.04), S: 0.028% (0.04), Si: 0.24%. The addition of this tiny amount of carbon made the steel much stronger and harder. Carbon steels account for about 90% of the world's steel production. They may be used for automobile bod­ies, appliances, machinery, ships, containers, and the structures of buildings.

Tool steels are special steels that are engineered to particular service requirements. These expensive alloys are exceptionally strong, hard, wear-resistant, tough, nonreactive to local overheat­ing. They contain tungsten, molybdenum, vanadium, and chromium in different combinations, and often cobalt or nickel for better high" temperature performance. They are used for machine tools, aircraft undercarriages, in buildings and bridges.

Stainless steels comprise any alloy steel that contains 10-30% chromium. The presence of chromium, together with the low carbon content, gives a remarkable resistance to corrosion and heat. Other elements, such as nickel, molybdenum, titanium, aluminum, niobi­um, copper, nitrogen, sulphur, phosphorus, and selenium, may be added for obtaining better corrosion resistance and other valuable properties.

V. Are these statements true or false? Correct the false statements.

1. There are many kinds of steels.

2. Three major classes are carbide steels, low-alloy steels, and high-alloy steels.

3. Carbon steel was used in building the Golden Gate Bridge.

4. Great strength, hardness and other valuable mechanical properties are obtained by the addition of a great amount of carbon.

5. Low-alloy steels are the most popular kind of steel.

6. Tool steel is used for producing automobile bodies, ships and spoons.

7. Tool steel is not cost-efficient.

8. Tungsten, molybdenum, vanadium, and chromium in different combinations may improve high-temperature performance of stain­less steel.

9. A remarkable resistance of stainless steel to corrosion and heat is achieved with the help of chromium and high carbon content.

Activity

II. Translate the passage into Russian. Use the dictionary if necessary.

Owing to the valuable properties of stainless steel its application may vary from spoons to bank vaults. This steel does not rust because of the interaction between its alloying elements and the environ­ment. Stainless steel contains iron, chromium, manganese, silicon, carbon and significant amounts of nickel and molybdenum. These elements react with oxygen from water and air and produce a very thin stable film of metal oxides and hydroxides, which may prevent additional corrosion because it limits the access of oxygen and water to the metal layers below. This film may not be seen without a power­ful microscope that is why steel seems stainless when it is in fact cor­roded at the atomic level.

In summary, stainless steel does not rust because it may form a corrosion product layer for the protection against further attacks of oxygen.

Дата добавления: 2015-11-14